Ropes for cranes typically include wire ropes formed by twisting steel wires.
The wire ropes are configured such that a core is disposed at the center thereof and a plurality of strands is twisted around the core. As such, each strand may be formed by twisting a plurality of fine filaments.
Because such wire ropes hold very heavy objects such as containers when used in a crane, they are designed to have at least a predetermined diameter or thickness in order to ensure strength or durability.
However, the wire ropes formed of steel are problematic because a tip load at the boom tip of a crane is drastically reduced due to the self-weight of steel wire rope in proportion to an increase in the height of a building. For example, in the case where a steel wire rope having a diameter of 36 mm is used in a 25-ton crane having a maximum work radius of 52.5 Mtr, the tip load of the crane is 2.3 tons. However, when a super fiber rope is used, the tip load is 6.8 tons, which is a 300% increase.
Furthermore, in the case where the capacity of the crane is increased to enhance the tip load of the crane in a super high-rise building, the self-weight of the crane increases. Accordingly, because a tower crane which is used in a state of being fixed to the outer wall of the building may have a considerable influence on the building, the outer wall of the building is designed to be much thicker, or severe problems may result if it is difficult to change the design of the building because of the features of the building.
As well, to avoid the problems caused by the use of such steel wire ropes, a plurality of cranes is conventionally used at different heights of the super high-rise building. However, the working time may increase due to lifting work undesirably decreasing work efficiency and lengthening the construction period.
Upon construction of a high-rise building, when conventional steel wire ropes are used in a crane, strength or durability is ensured but the self-weight thereof is large and thus the tip load of the crane is remarkably decreased. In order to increase the tip load of the crane, the capacity of the crane should be increased. In this case, however, the weight of the crane body may increase, undesirably placing additional burdens on the design of the building. Furthermore, as the height of the building increases, equipment is made complicated, and lifting efficiency may decrease, undesirably generating a variety of problems including a long construction period.
Because of such problems, ropes made of synthetic fibers (in particular high-strength super fibers) conventionally employed in different end uses may be utilized. Conventional synthetic fiber ropes are disadvantageous because the circular cross-section thereof is deformed into a flat oval shape due to the lifting load when wound on the drum of a tower crane, and also because the deformation shape is non-uniform, making it impossible to form an aligned winding. In the case where such an aligned winding is not formed, there may occur a phenomenon in which the rope is caught between the underlying rope turns of the non-aligned winding upon lifting high loads by the crane. When the caught rope is released between the underlying rope turns during unwinding at high speed, an impact may be applied to the rope, undesirably causing problems of the lifting object swinging or falling. Where such an impact may accumulate, the lifetime of the rope may be decreased, and the rope may be damaged attributable to loads intensively applied to a specific portion thereof. Moreover, irregular winding on a crane drum may increase the winding volume, undesirably generating a variety of problems including causing friction with a portion close to the drum to thereby directly break the ropes.